1. Field of the Invention
This invention involves a technique for forming optical fibers, and more particularly for forming an optical fiber with an optimized index of refraction configuration using differential mode-group delay measurement.
2. Description of the Prior Art
The advantages that accrue when information is transmitted using electromagnetic waves in the "optical" region of the spectrum (0.4.mu.-2.0.mu.) have now been firmly established and well-accepted. These advantages include high bandwidth, immunity from electrical interference, and electrical isolation from connected terminals. While initial inquiry into the feasibility of optical transmission included consideration of numerous different transmission media, research and development performed during the past decade has established the viability of glass fibers as a primary transmission medium. These fibers generally comprise a core region where most of the transmission occurs and a cladding region of lower index of refraction where less, and in many configurations a negligibly small amount of transmission occurs. The cladding, by virtue of its lower index of refraction, contributes to the concentration of optical power within the fiber core and its transmission through the core without escaping to the outside environment. Additional material may serve the function of protecting the fiber, and many include additional glass layers and/or various plastic layers formed about the fiber.
The transmission characteristics of the fiber are very much dependent upon the fiber's index of refraction configuration, and especially on its variation in the radial direction. So, for example, outer regions of the cladding must have a lower index of refraction than inner transmitting regions of the fiber if effective waveguiding is to take place. Likewise, the fiber will transmit in many different electromagnetic configurations, or modes, or in only a single mode, depending upon the core size the index of refraction configuration, the wavelength of light being transmitted, as well as other parameters such as launching angle and curvature of the fiber. Never-the-less, as a general rule, a single mode fiber will have a relatively small diameter core, of the order of 10 microns, and a relatively thick cladding, of diameter on the order of 100 microns, while multimode fibers have a much larger core on the order of 50 microns, and a cladding approximately 25 microns thick.
Multimode fibers in which the transmitted radiation can exist in many different electromagnetic configurations generally have a radially graded index of refraction, although certain step indexed radial configurations will also transmit in the multimode configuration. The radial gradation is used to minimize the "mode dispersion" pulse broadening effect which is associated with multimode fibers. This pulse broadening is due to the different transmission paths associated with each of the modes. For example, in step index multimode fibers, lower order modes are transmitted essentially down the center of the fiber, while higher order modes are transmitted down the fiber along paths which oscillate back and forth from the center of the fiber core to its periphery. The longer optical path lengths associated with higher order modes generally result in longer transit times for these modes. A given pulse transmitted through the fiber is transmitted in a combination of many possible modes. Those portions of the pulse which are transmitted in higher order modes will arrive later than those portions transmitted in lower order modes, by virtue of the longer path length associated with the higher modes, and consequently the width of the pulse will be significantly broadened, with a concomitant lowering in bandwidth.
In a graded index multimode fiber, the radial gradation, by virtue of its lower index of refraction at larger radii, results in increased velocity for the higher order modes which spend more of their time at the periphery of the fiber core. This increase in velocity tends to compensate for the greater path length of higher order modes and approximately equalizes the transit time associated with the various modes thereby minimizing the mode dispersion effect.
The exact nature of the radial gradation and its effect on lowering mode dispersion is clearly a critical element in the design and function of an optical fiber. While theoretical studies predict various preferred index configurations, the art is still at a stage where direct measurement is desirable for direct determination of the mode dispersion properties of an optical fiber. One of the more promising techniques for measuring mode dispersion involves the selective excitation of particular mode groups within a fiber and the determination of the transit time associated with the excited subgroup of modes. Numerous subgroups within the fiber are excited individually and the mode dispersion properties of the fiber are thereby determined.
It is clear that the particular excitation technique used must effectively excite only a relatively small subgroup of modes if the technique is to be successful. One excitation technique involves the use of a beam focused by an appropriate lens and launched into the fiber. [See, for example, L. G. Cohen, Applied Optics 15, 1808 (1976), and L. Jeunhomme, et al Applied Optics 17 463 (1978)]. However, in order to obtain an acceptably small angular spectrum of rays, the lens must be placed far from the fiber. In addition, the technique is very sensitive to beam characteristics which are difficult to determine and to the physical characteristics of the fiber end face.